Considering a specific electrolytic reaction, discussed by way of example only, the overall reaction of water splitting, 2H2O→2H2+O2, produces O2 and H2 gases as end products. Water splitting is one of the simplest ways to produce high purity hydrogen. Although the current efficiency of water electrolysis lies in the range of 50-70%, the current cost of hydrogen gas produced by this method is in the range of about $20-30/GJ (assuming $0.05/kWh), compared to about $6-12/GJ for hydrogen gas produced via natural gas reforming and coal gasification.
For water splitting, and many other reactions, gases need to be kept separate for later individual use and to avoid production of an explosive gas mixture. There are several approaches to the design of devices that can maintain separation of two or more gases during electrolysis, for example the use of a membrane to separate electrode compartments or chambers. This also minimizes cross-over of dissolved gases from one electrode to be recycled at another electrode.
Although gases can be separated, new issues arise with these technologies, e.g. cost, mechanical properties, high resistance through the membrane, and in the case of water splitting ultra pure water is needed for proper operation.
As another example, alkaline zero gap electrolysers using OH− conducting membranes are also being considered. In a traditional alkaline electrolyser, where a diaphragm is the only separator, bubble formation inside and between the electrode and the separator is the major cause of transport resistance. A number of suggestions on bubble management have been made, e.g. use of mechanical circulation of the electrolyte and use of (stable) additives to reduce surface tension of the electrolyte so bubbles can more easily leave the system.
By way of example in relation to water splitting, one of the features of the O2 evolution reaction is that the dissolved oxygen concentration at the electrode has to build up to a level sufficient to nucleate and form small, high-pressure bubbles. According to Laplace's equation: P=2γ/r, where P is pressure in the bubble, γ is the surface tension and r the radius of the bubble, near the surface of an electrolyte, O2 bubbles with 0.1 μm radius need to have a pressure of 14 atm at 25° C. The concentrations required not only produce overpotential at the electrode, but also represent a very reactive environment that challenges the long term stability of many catalysts for water splitting, as well as for other electrolytic reactions.
Reports have described efforts to improve cell efficiency, such as for water splitting, by addition of sacrificial agents or co-catalysts, modification of catalyst crystal structures and morphology, and specific surface area. Also, there have been attempts to separate gases using different flow streams of the electrolyte in a planar microfabricated device, but the device efficiency was not high.
Improved removal of gases, such as O2 and H2, from a cell before bubbles are formed has not yet been suitably or sufficiently addressed. Traditional gas diffusion electrodes (GDE) of the type used in fuel cells have a tendency to continue to form O2 bubbles, for example when operating as water splitting devices. Moreover, these electrodes are not stable under water oxidation (WO) conditions, with carbon being rapidly oxidized at the potentials involved in water oxidation.
The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention.